Silicon photonics is an emerging technology that enables fabrication of fast, low-cost and high-density photonic devices for a variety of uses and applications, notably in relation to optical network switching. Photonic switch fabrics, which are capable of switching optical signals without conversion to the digital domain, are an area of particular interest.
A silicon photonic switch typically incorporates a number of optical waveguides of varying lengths, multiple optical switch cells, and multiple waveguide crossings having intersecting waveguides.
A silicon photonic device, such as a silicon photonic switch, can be fabricated using a commercially available “silicon-on-insulator” (SOI) wafer composed of a thin layer of silicon (typically 150-350 nm thick) disposed on a layer of buried oxide (e.g. silicon dioxide) which is, in turn, supported on a silicon substrate typically several hundred micrometers thick. The fabrication of photonic devices requires a number of steps that include, for example, chemical vapor deposition and patterning of layers to from waveguides using lithography and etching.
The commercially viable implementation of a silicon photonic switch fabric imposes demanding requirements in terms of optical performance (e.g. insertion loss, crosstalk) and optical component density. In a silicon photonic switch matrix, the insertion loss can be attributed primarily to a small number of sources: fiber-to-integrated-circuit coupling loss, waveguide scattering loss, switch cell insertion loss, and waveguide crossing loss. Waveguide crossings are necessary in switch fabrics. In large switch fabrics, there may be a significant number of waveguide crossings which cause losses and crosstalk. In silicon photonics, the waveguide scattering loss is relatively high, i.e. approximately 2 dB/cm for strongly confining strip waveguides. In a large switch matrix, therefore, long optical paths can cause a large scattering loss to accumulate, and many crossings can cause crosstalk to accumulate.
There is accordingly a need in silicon photonics for lower loss and low crosstalk yet broadband waveguide crossings which enable different parts of a very large-scale photonic circuit to be interconnected in complex ways.